When water is present in an oil, fuel or other solvent, the preferred method of removal is through the use of a hydrophobic filter screen that prevents the passage of water. Such a screen can become covered with water, and the covering prevents the passage of oil or fuel through the filter. Water may then be removed by backflushing or sweeping the surface with a flow to carry away the trapped water. However, the need for backflushing imparts an additional function that suspends the action of the filter for a period of time.
Other methods have used hydrophobic prefilters, but these suffer from the same need to backflush or sweep with a flow to remove the water on the filter surface.
In order to obviate the need to backflush, hydrophilic filters are often chosen for the first filter in a system. The hydrophilic filter will allow the passage of water into its interior, where the particles are absorbed onto the filter medium surface and coalesce into larger globules of water. These then eventually break free and pass into the gap separating the first filter from the second filter. There, a stream of oil or fuel carries away the water that has passed the first filter. However, when the oil or fuel being filtered contains an emulsifying agent, the particles of water will remain suspended and pass through the first filter without coalescing, and continue on to the second filter. The emulsified water will then pass through this last filter under pressure, and continue to contaminate the oil or fuel.
In this typical construction of the prior art used to remove water from jet fuel, a conventional fuel-water separator is usually comprised of two different filter cartridges. The two cartridges are arranged in series. The first is a water-coalescing cartridge, and the second is a water-separating cartridge. This latter cartridge is hydrophobic and operates to exclude water as described. Fuel contaminated with water passes through the coalescing filter cartridge first, which has a pore size range of 1 μm to 100 μm, preferably in the range of 1 μm to 20 μm. The coalescing cartridge usually has a pleated design or a string wound design utilizing hydrophilic material, such as cotton. Fine water droplets are absorbed by the filter fibers due to their hydrophilic surface property. As more and more water is absorbed in the filter cartridge, agglomeration occurs and larger water globules (greater than 100×100 mesh typically used) are formed. The jet fuel flowing through this first cartridge then carries these away. Then the jet fuel containing water globules flows into the separation cartridge, which is made from 100×100 mesh PTFE screen. In the prior art, the mesh size must be this large to prevent the buildup of water on the surface, which will occur with smaller mesh sizes. The jet fuel freely passes through the screen, but, due to its hydrophobic surface property, the PTFE screen retains the water globules and prevents their passage. The retained water globules then settle down to the bottom of the water collection chamber.
Surfactant fuel additives are often added to jet fuel for the purpose of cleaning the aircraft fuel system and allowing the engine components to operate more effectively and efficiently at higher temperatures. One particularly useful additive is SPEC-AID 8Q462, as sold by BetzDearborn In., Trevose, Pa., which is known as a +100 additive because it allows engine operation temperature to be increased by up to 100 degrees Fahrenheit. However, the side effect of surfactant fuel additives is that they break down the water droplets to much smaller sizes (1 μm to 10 μm), forming a stable water emulsion in the jet fuel. Each water droplet is surrounded by surfactant, the molecules of which consist of a hydrophilic head functional group (hydrophilic head) and a hydrophobic tail functional group (hydrophobic tail). The hydrophilic heads of the surfactant molecules attach to the water droplet and the hydrophobic tails face outward, where they are solvated by the jet fuel and form a stable emulsion. Very small droplets of water bound by surfactant thus characterize this emulsion. Since the surfactant-coated water droplets are thus hydrophobic at their surface, they will not be absorbed in the hydrophilic coalescing filter cartridge of the prior art. Therefore, there will be no water coalescing effect in the coalescing filters. Consequently, the jet fuel and the fine surfactant-bound water droplets freely pass through the first filter without coagulation, remain dispersed in the flow stream and reach the PTFE screen filter cartridge, where, due to the much larger pore size of the screen, they pass through and continue to contaminate the fuel.
In the instant invention, the filter medium is chosen to be hydrophobic in contrast to the accepted prior art. However, since the water molecules are bound with surfactant, and are now functionally hydrophobic, the water is not repelled by the hydrophobic filter medium, but rather passes into the filter. Because the tail of the surfactant molecule is hydrophobic, it is attracted to the surface of the hydrophobic filter medium. At the surface of the hydrophobic filter, the surfactant-bound water attaches and waits until a larger build-up occurs. As the surfactant-bound water molecules pass into and build up on the surface of the hydrophobic filter, the water agglomerates, breaking the boundary of the surfactant. The coagulated water then passes out of the filter into the stream between the first and second filter.
Similar to the conventional fuel-water separator, the instant invention is also comprised of two filter cartridges: A water coagulation cartridge and a hydrophobic water separation cartridge. But here the similarity ends. The water coagulation cartridge of the instant invention is a hydrophobic depth filter cartridge. The filter medium can be nylon, polyester, polyvinylidene difluoride or polypropylene. As discussed above, the surfactant-coated water droplets have a hydrophobic surface when surfactant additives are present in the oil or fuel. As the oil or fuel and the now “hydrophobic water droplets” flow through the hydrophobic filter cartridge, the “hydrophobic water droplets” attempt to be absorbed by the hydrophobic filter fibers and become contained within the filter. As more water droplets are absorbed in the cartridge, multi-layer water/additive globules are formed and, when they become large enough, are carried away by the oil or fuel flow. A globule of water/additive is comprised of multiple water droplets. Its size is usually 5 to 10 times larger than that of a single emulsified water droplet, which would typically be in the range of 1 μm to 10 μm. This action within the filter greatly reduces the degree of water emulsification in the oil or fuel. However, the globules are still in the range of micron sizes and do not settle down easily. The second function of the water coagulation filter is to separate dirt, bacteria, and other suspended solids from the oil or fuel.
Next the oil or fuel and water/additive globules flow to the water separation filter cartridge, which is formed with a hydrophobic membrane (e.g., PTFE) of 0.1 to 5.0 μm pore size, which is approximately three orders of magnitude smaller than used in prior art technology. Use of a filter with such a small pore size with the technology taught in the prior art will result in rapid blocking of the filter surface by water and shut down of the oil or fuel flow. A bypass-flow or sweeping-flow is maintained on the membrane surface at the feed side. The sweeping-flow is used to sweep the membrane surface with high shear motion and to carry the suspension away from the filter surface, while the fuel component of the liquid (e.g., jet fuel), or the oil, penetrates into the membrane pores under pressure. Examples of suitable filter designs include spiral wound module cartridges, tubular cartridges, and hollow fiber cartridges. The desirable sweeping-flow rate is generated by a pumping force across the membrane surface at 0.05 to 3.00 meters per second and the desirable ratio of sweeping-flow rate to the fresh-feed rate is 5:1 to 1:10 by volume.
When oil or jet fuel has surfactant added to it, three things are needed to successfully separate water from the oil or jet fuel with surfactants, particularly when used with surfactants known as “+100 additive”. These are
1) hydrophobic membrane
2) sub micron pore size (e.g., 0.1 to 5.0 μm), and
3) sweeping-flow.
Theoretically, hydrophilic membranes can be used for the separation filter in this type of application. However, water droplets are not always completely coated with the hydrophobic substance (additive). Therefore, uncoated water droplets can freely pass through the pores of a hydrophilic membrane. Using a hydrophobic membrane ensures that the uncoated water droplets cannot go through its pores. Since the surface energy of the coated water droplets is similar to jet fuel, the coated water droplet and jet fuel should have similar wettability on the hydrophobic membrane surface. In this case, separation is only controlled by the given pore size of the hydrophobic membrane. The membrane rejects any suspended particle with greater size than the membrane pores. Studies by the inventor have shown that a 0.1 to 5.0 μm PTFE membrane gives desirable water rejection rate and permeate flow rate. Due to the hydrophobic property of the coated water droplets, they favor remaining on the hydrophobic membrane surface. If a water boundary layer is formed on the membrane surface, a certain amount of water will bleed through the membrane under pressure. To solve this problem, a sweeping-flow of oil or fuel is formed on the membrane surface to sweep away the water droplets. With a 0.1 to 5.0 μm PTFE membrane, it is important to maintain a differential pressure that does not exceed 100 psi between the feed solution and the permeate, in order to prevent bleed through of water at the membrane filter. The inventor has also found that the temperature should not exceed 130 degrees Fahrenheit in order to prevent water from vaporizing, passing through both filters and then condensing in the clean fuel.
After exiting the water separation cartridge, the sweeping-flow stream (or concentrate) carries the concentrated emulsified water droplets and then enters a water-settling chamber. In this chamber, a relatively quiet environment is maintained. Fine water droplets agglomerate and form a heavier phase within the chamber. As more water droplets agglomerate in the heavier phase, water emulsion breakdown occurs, and free water is formed at the bottom of the water-settling chamber.
The water separation filter cartridge (PTFE membrane cartridge) works well by itself without the coagulation filter cartridge, if the water concentration is below 0.5% in the feed. However, the permeate flow rate can significantly drop if the water concentration is higher than 1% because the water forms a layer that blocks the surface of the filter. To make an oil or fuel filter commercially practical, it must pass a test with a 3% water concentration in jet fuel and a permeate flow flux of at least 0.5 gallon/min./sq.-ft. of membrane area. The hydrophobic coagulation filter cartridge is a critical component to ensure adequate permeate flow rate with 3% water concentration in the feed. If no prefilter is present, there is a buildup of water that blocks further oil or fuel from passing through the filter. When there is a hydrophilic pre-filter, filtration is excellent, so long as there is no surfactant present to emulsify the water. However, when surfactant is present, the hydrophilic filter allows passage of the water that is emulsified, which then goes through the second filter, since there has been no coalescence.
U.S. Pat. No. 6,042,722 to Lenz teaches a single separator for removal of water by specific gravity from various fuels, including diesel and jet fuel.
U.S. Pat. Nos. 6,203,698 and 5,916,442 both to Goodrich teach the use of hydrophobic filter media to reject water from passage through the filter.
U.S. Pat. No. 5,993,675 to Hagerthy teaches the use of microfibers, which are impervious to the passage of water, but which allow the fuel to flow through.
U.S. Pat. Nos. RE37,165, 5,766,449 and 5,507,942 all to Davis all teach a single filter, which is hydrophobic so that it rejects water penetration.
Although some of these methods rely on a hydrophobic material to reject water, all of these methods utilize a single filter and none of them utilizes the hydrophobic filter to capture and coalesce surfactant-bound water. They function merely by rejection of normal size water droplets, and would be inadequate for rejection of emulsified water.
U.S. Pat. No. 4,988,445 to Fulk teaches the use of multiple spirally wound filters used in two stages. Fulk teaches a “means for enabling concentrate from said first stage module to pass directly to said second stage modules without passing through a pump; [a] means for forcing said feed stream through said first and second stages; and [a] means for recycling a portion of the concentrate from said second stage to said first stage.” U.S. Pat. No. 6,146,535 to Sutherland teaches the use of hollow microfibers for phase separation, by exclusion of the aqueous phase through pore size hydrophobicity. Neither of these patents teaches the use of a hydrophobic first filter for the removal of surfactant-bound water through the use of the functional group properties of the surfactant, as is the case with the present invention.
Among other patents, several are of particular interest in evaluating the present invention. For instance to U.S. Pat. No. 4,814,087 to Taylor teaches a number of devices that are able to remove suspended water from fuels. Among these are coalescing devices and electrostatic precipitators. These coalescing devices become filled with water during operation and must be maintained carefully to prevent water from being pumped with the fuel to the point of use.
U.S. Pat. No. 4,372,847 to Lewis teaches the use of a cartridge for filtration that comprises a coalescing stage and a separating stage. This invention is specifically geared to separation of emulsified liquids. It functions through the formation of coalesced droplets that form due to a different specific gravity at the coalescing stage and remain free for removal at the second hydrophobic separating stage.
U.S. Pat. No. 4,814,087 to Taylor teaches a single stage cross-flow hydrophobic separator comprised of a microporous material. Cross-flow is used to clear the water from the separator.
U.S. Pat. No. 5,149,433 to Lien teaches the use of two spirally wound filters in series, whereby the second filter only functions for the removal of water from fuel if the first one fails. Cross-flow is used for the first filter to sweep away water as it accumulates.
U.S. Pat. No. 4,846,976 to Ford teaches a filtration system for a water-containing emulsion that is comprised of two stages, both comprised of hydrophobic microfilters. A backwash accomplishes cleaning of the first microfilter. While this uses hydrophobic material, this invention serves to remove small quantities of emulsified oil and fat from the water, thus providing clean water for disposal, rather than removal of water from the hydrocarbons.
U.S. Pat. No. 5,443,724 to Williamson et al., teaches the use of two filters, the first being a coalescing unit and the second being a separating unit. Coalescence is accomplished by a choice of physical shape of packing material for a critical wetting surface energy “intermediate the critical wetting surface tension of the discontinuous and continuous phases”.
The present invention differs from these examples of prior art in distinct ways. The principal function of the present invention is the removal of emulsified water from hydrocarbons, such as oil or fuel. The present invention utilizes two stages of filtration to accomplish the goal of removal of water from oil or fuel. Much of the prior art utilizes single stages that are less effective at removal and cannot remove emulsified water, as it would pass through their filters. Other two stage filtration systems also suffer from the inability to separate emulsified water from the oil or fuel.
The present invention functions by providing a coalescing surface which is near the surface energy of the hydrophobic tail end of the surfactant molecule, whose head end is attached to a water molecule. Due to the attraction of the matching coalescing surface and the tail end of the surfactant, there is a build-up of bound water molecules to form and agglomerate, which agglomerate is then swept through by the oil or jet fuel. Once in the flow between the first and second stages, the agglomerated water is swept away by the sweeping-flow.
The present invention differs from U.S. Pat. No. 4,846,976 to Ford, in that Ford essentially teaches the opposite. I.e., removal of small quantities of dispersed, surfactant-bound, fats and oils from water by hydrophobic filters. This would imply that to do the opposite, that is, to remove dispersed, surfactant-coated water, one would require hydrophilic filters (as is the case for conventional two-stage filters).
The present invention differs from U.S. Pat. No. 4,372,847 to Lewis, since Lewis utilizes specific gravity for the coalescing function.
The present invention differs from U.S. Pat. No. 4,814,087 to Taylor, in that Taylor uses a single stage hydrophobic filter and removes only dissolved water. Taylor does mention that coalescers may be used for removal of suspended water, but does not describe a method or apparatus for so doing.
The present invention differs from U.S. Pat. No. 5,443,724 to Williamson et al., in that Williamson et al. removes only particles greater than 0.01 inches and requires water droplets to pass by the surface of a second filter under gravitational force, while the present invention utilizes a high rate of flow forced by pumping. The settling velocity of Williamson et al. is shown in TABLE I.
TABLE 1Water Droplet DiameterWater Droplet Settling(inches)Velocity in Fuel (m/s)0.010.0030.0150.0060.020.0110.0250.0150.030.0190.0350.0230.040.0280.0450.0330.050.038
For the present invention, wherein the size of the agglomerated particles is less than 0.004 inches, the flow velocity over the selected range of ratios of sweeping-flow to fresh-feed flow ranges between 0.05 and 3.00 meters per second, as seen in TABLE II below.
TABLE IIRatio of Sweeping Flow toSweeping Flow Velocity onFresh Feed FlowMembrane Surface (m/s)10%0.0533%0.1950%0.2967%0.39100%0.60200%1.16300%1.75400%2.33500%3.00
Thus, it is readily seen that the sweeping-flow velocity of the present invention, at a minimum of 0.05 meters per second is substantially higher than the gravitational flow of Williamson et al., which reaches a maximum at 0.038 meters per second for a droplet of 0.05 inches in diameter. The droplet size of the present invention is 0.004 inches at a maximum, substantially smaller than the smallest water droplet of Williamson et al, which would have a velocity of only 0.003 meters per second under gravitational force.
Williamson et al. utilizes physical shape of the packing material for coalescence in the fashion of a baffle, and further that Williamson et al. specifies that the coalescer must allow wetting by the fuel, but not by the suspended water (discontinuous liquid phase). In the present invention, the coalescer is specifically hydrophobic to match the hydrophobic tail of the wetting agent, and its surface tension is thus near to or lower than that of the surfactant-bound water. Thus, the surface energy of the coalescing cartridge of the instant invention has no relationship to the surface tension of unbound water, but is specifically wet by the suspended water (discontinuous phase).
According to Williamson et al., the coalescing element must have a surface energy (or critical wetting surface tension) which is greater than the surface tension of the continuous liquid phase. In fact, Williamson et al. specifically requires that the surface energy be intermediate the continuous phase (fuel) and the discontinuous phase (water). Since jet fuel is approximately 23 mN/m and water is 72.5 mN/m, this would lead to practice of the art in Williamson et al. with a coalescer of approximately 48 mN/m, which is clearly much greater than for the hydrophobic materials of the present invention, which are typically around 30 mN/m or less.
In the instant invention, the coalescing element may have a surface energy lower than the surface tension of the continuous phase, and is preferably as close as possible to the surface tension of the continuous phase. The surface tension of the discontinuous phase is wholly irrelevant, since it is bound with surfactant molecules, whose very function is to transform the discontinuous phase into a material having a surface tension that is very close to the continuous phase.
The present invention offers significant features and advantages over the above prior art devices and methods.
Accordingly, a feature and advantage of the present invention is that it provides a method and a device to remove water (up to 5%) in oil or fuel, and to obtain a clean output of oil or fuel with less than 5 ppm of water, while maintain a high flow rate not possible with the prior art.
A further feature and advantage of the present invention is that instead of coalescing water from oil or fuel in between the first stage and the second stage as in a conventional fuel filter design, this invention uses a hydrophobic depth filter and a PTFE membrane filter to separate and concentrate contaminated oil or fuel. The concentrate goes to a settling chamber after the two filtration stages. Free water settles down in the chamber.
A anthoer feature and advantage of the present invention is that the coagulation filter cartridges and water separation filter cartridges may be installed in one filter housing with multiple chamber design.
Still another feature and advantage of the present invention is that the internal circulating pump design eliminates the need for a working tank, which is necessary for common cross-flow filter systems.
Still yet another feature and advantage of the present invention is that this invention solves the problem of inefficiently and ineffectively removing emulsified water from oil or fuel with surfactant additives when using a conventional fuel-water separator.
Still a further feature and advantage of the present invention is its ability to overcome the common problem of the filter becoming dry and requiring change-out, when it is idle after use. This will occur where hydrophilic materials are used for the filter, as they will crack when they dry out. Synthetic fibers do not suffer this problem and are typically hydrophobic.
These and other features and advantages of the present invention will become more apparent to one skilled in the art from the following description and claims when read in light of the accompanying drawings.